The present invention relates to storage systems.
Conventional data storage systems include one or more storage devices connected to a controller or manager. As used herein, the term “data storage device” refers to any device or apparatus utilizable for the storage of data, e.g., a disk drive. For explanatory purposes only and not as an intent to limit the scope of the invention, the term “disk drive” will be used throughout this document instead of the term “data storage device.”
A logical volume manager (also called a logical disk manager) can be used to manage storage systems containing multiple disk drives. The logical volume manager configures a pool of disk drives into logical volumes (also called logical disks) so that applications and users interface with logical volumes instead of directly accessing physical disk drives. One advantage of using a logical volume manager is that a logical volume may span multiple physical disks, hut is accessed transparently as if it were a single disk drive. These logical volumes appear to other components of the computer system as ordinary physical disk drives, but with performance and reliability characteristics that are different from underlying disk drives.
The logical volume manager divides a physical disk drive into one or more partitions which are also known as extents or subdisks. Each logical volume is composed of one or more partitions and each partition is typically defined by an offset and length. Because of the overhead inherent in managing multiple partitions, conventional systems normally have severe limitations on the number of partitions that can be formed on a physical disk drive. The practical limit in conventional systems is normally less than 100 (and often less than 10) partitions on a single disk drive. Due to the nature of the data structures and algorithms used by conventional volume managers, the maximum number of partitions or subdisks permitted to a logical volume in conventional systems is usually much less than 5000. In the simplest case, the disk manager forms a logical volume from a single partition. In more complex cases, the disk manager may form logical volumes by concatenating multiple partitions.
Each partition can, and typically does, have a different length. When a logical volume is no longer needed, its partitions are deleted so that space on the disk drives is made available for another partition to be created. However if a new partition is larger than the available space, then the space cannot be reused for the new partition. If the new partition is smaller than the available space, then a portion of the free space will be used and an even smaller piece will remain free. Over time, this results in many small pieces of free space that cannot be reused. This problem is often referred to as “fragmentation.”
Traditional approaches to fragmentation problems often introduce other problems into the system. For example, one traditional solution is to move existing partitions together so that the system free space is in one piece. However, this solution could be quite expensive since a significant amount of existing data may have to be moved to place all the partitions together. Moreover, the corresponding data may have to be locked during the move to prevent data inconsistencies from occurring. As a result, this solution could reduce or prevent the availability of data to users during the data move.
Load balancing is another function that should be addressed by the logical volume manager, since the manner in which data is distributed among disk drives may cause load balancing problems. A disk drive can usually service only one I/O request at a time. Requests received at a “busy” disk drive are stored in a queue for later processing, usually in the order received. If one disk drive is accessed more than other disk drives, the queue for accessing data from the busier disk drive becomes longer, and accordingly, the wait also becomes longer. This may result in some disk drives being overloaded while others remain idle or lightly loaded.
Solutions have been proposed to solve this load balancing problem but with limited success. A heavily accessed logical volume may be striped over a number of disk drives to distribute the load. However, the number of partition concatenations to stripe across must typically be chosen when the logical volume is allocated. This requires knowing ahead of time that a set of data is going to be heavily accessed, and presumes that the access pattern will not change over time. Because of changing access patterns, it is usually very difficult to predict optimal striping patterns ahead of time.
Another possible solution is to gather statistics about the frequency in which different logical volumes are accessed, and then reallocate multiple logical volumes to put less frequently accessed logical volumes on the same physical disk drives as more heavily accessed logical volumes. Logical volumes may also be reallocated to be striped over more disk drives. Deciding how to reallocate, however, is usually a labor intensive administrative task with conventional systems. Once data has been stored, it is normally quite expensive to move that data around. The data is either made unavailable or significant overhead must be incurred to coordinate normal accesses with the movement of the data. In addition, changing the number of disk drives for striping normally requires recopying of the entire logical volume.
A disk drive can be added to a system to increase the amount of available storage. Typically, new data is stored in the new disk drive, rather than moving existing data to be stored in the new disk drive. It may be necessary in some circumstances to add disk drives to support more I/O operations rather than to just provide more storage. However, adding a disk drive for this purpose raises many of the same problems associated with load balancing. For example, when first added, a new disk drive is like a device that has been misconfigured to be idle and needs data from existing logical volumes to be moved to it.
The foregoing problems of the conventional systems are further exasperated by systems containing many disk drives (e.g., a thousand or more disk drives). This is due in large part to the amount of manual administration required in conventional systems. In conventional systems, the functions of configuring, addressing, and administering logical volumes and disk drives are normally performed manually by an administrator who must make choices as to the proper configuration to employ. When a large number of disk drives and/or logical volumes are used, this manual administration becomes more and more difficult. Thus, existing systems are prone to human error and their structures (administrative and data) do not scale well beyond a certain number of disk drives.
One approach to address the above problems is to form a logical volume using extents from multiple disk drives. In this approach, each extent is small compared to the size of a logical volume or disk drive. Fragmentation is reduced or eliminated because extents having the same size or sizes are allocated across the disk drives in the storage system. Allocations of the extents can be made along boundaries that correspond to the number of contiguous pieces being allocated. The portions of disk drives that form a logical volume are spread out as evenly as is practical so that two pieces on the same disk drive are far apart in the address space of the logical volume. Thus, I/O load is spread evenly over many disk drives.
The present invention provides a method and mechanism for implementing variable sized extents for a logical volume or file. Instead of using extents which all have the same size to form a logical volume, the present invention permits a logical volume to include extents having different extent sizes. In an embodiment, a relatively small extent size is used for extents allocated for the first portion of a logical volume, which increases to a larger extent size for extents allocated to a second portion of the logical volume, which again increase one or more times for extents allocated for later portions of the logical volume.
Further details of aspects, objects, and advantages of the invention are described below in the detailed description, drawings, and claims.